In the middle of last week a lot of people, asked for their reaction to the carnage in West Beirut, said that, disturbing as it was, they still didn't have enough facts to render a judgment. I thought this was an evasion. By then we knew as much as we needed to in order to reach a judgment about (1) the implications for Israel of this monstrous event and (2) the imperatives that flowed from it.

The burdensome truth -- that certain actions were now required -- seemed to be understood by great numbers of Israelis before it was even grudgingly conceded by some in this country who describe themselves as "defenders of Israel." But more on them in a moment. Israelis understood at once, I think, that this was something they could not live with or leave unattended. To do so would be an act of self-destruction. That is why I am certain they will respond.

What is it that we knew almost at once? We knew that the government of Israel, along with representatives of the United States and other countries and various parties to the Lebanese conflict, having negotiated the departure of the PLO forces from Beirut, had in some degree undertaken, if not to protect, at the very least not to harm, the civilians and dependents left behind; that after the murder of Bashir Gemayel, Israeli forces moved into West Beirut on the claim that they were needed there to maintain order; that far from maintaining order, which would surely have implied keeping the bereaved and raging Christian militias and the Palestinians apart, they used their authority to usher the massacre makers into the refugee camps and stood by while the murders took place.

This is what we know. Some people have responded to it by pointing out with great irritation that you would never judge from all the fuss being made that it was actually Lebanese gunmen, not Israeli soldiers, who did the slaughtering. But to say that is to imply that this one fact negates or renders irrelevant the other, which is not true. It is also to suggest that somehow the Israeli involvement does not itself raise fundamental questions of moral responsibility. Israelis know otherwise. That is why they are protesting.

While no one is seeking to justify what happened in those camps in West Beirut, some, especially in this country, do seek to account for it in ways that shade over into extenuation. President Reagan himself the other day evoked one of these theories, when he employed the reigning and mischievous clich,e about the "quagmire" into which Israel is supposed to be about "to sink more deeply" in its pursuit of its objectives in Lebanon. The quagmire idea -- a shapeless, treacherous, dark and deceptive marshland, where those who enter lose control of their fate, being drawn almost involuntarily into actions they neither intend nor understand nor even can bring to a halt -- is in no way an accurate metaphor for what happened. It is, in addition, inimical in its implications to the whole Israeli spirit.

At each step of the way, Gen. Sharon and Prime Minister Begin knew what they were doing. They acted out of choice. They were not "drawn" into some half-lit swamp by an enemy confounding and outwitting them into ever greater and more disastrous engagement. They would never be allowed to be. Israel's very survival is a tribute to its insistence on conducting its business in precisely the opposite manner. Wakefulness, self-reliance, accountability, the exertion of will -- these are its worshiped secular values. An often even belligerent assumption of responsibility for its own destiny and actions, sometimes seen by others as arrogance, is in fact an integral part of its post-Holocaust credo.

This is one reason the Israelis will press on to determine who was responsible for facilitating the massacres and hold these people accountable for their actions, inevitably, I think, kicking them out. Israelis will be unable, as individuals and as a society, to rest or to consider themselves whole or right until they do. They have no use or even toleration for quagmire theories of history and human behavior. They know where these lead.

A second explanation/extenuation points to the terrible acts of violence and wanton cruelty--the other massacres--that preceded and, in certain undeniable respects, called forth this one: Palestinians have committed hideous crimes of terrorism against Israelis and Lebanese Christians and also against each other in recent memory. Hatred and revenge have their roots in countless atrocities forgotten now by us, perhaps, if we ever even noted them, but understandably kept keen and vivid in the minds of their surviving victims.

It still won't do. For this latest round of victims -- men, women and children -- were evidently randomly chosen and gunned down for a single indisputable offense, and it was this: they were Palestinians. They were murdered solely on that account. The tableau of corpses and shrieking survivors and bulldozers handily brought in to do the mass body removal -- this is something that produces volcanic emotions in Jews, as well it should; it is something that must be faced up to, repented, exorcised. Whoever and whatever made it possible for the Israeli authorities to countenance the actions that led to this tableau will have to be exposed and rejected by the Israelis themselves. They will be.

In some sense, the PLO terrorists' real revenge on the Israelis may be measured as much by whatever success they have in corrupting Israeli sensibilities and emotions as in ruthlessly killing Israeli citizens and friends. To harden those feelings so that Palestinians as such -- as a people, as a class -- are regarded by Israelis as suitable objects of vengeance would be to assault Israel's central idea: the morallclaim it makes to nationhood and survival and to the concern and consideration of civilized peoples. Israel, in other words, cannot merely look the other way and let the moment pass, except at the cost of transforming itself and its own identity. A people so notoriously mindful of threats to their survival will recognize this situation as one. It is not the sophists and alibi makers in this country, but rather the agitated, demanding protesters in Tel Aviv who are the true "defenders of Israel."